fix(itinerary): fix route optimization reactivity and replace api key provider dropdown with AI LLM providers

- fix optimizeDayOrder() dual-update: directly set days[dayIndex].items + days before
  saveReorderedItems() so it reads the correct post-optimization order synchronously
  (Svelte 4 batches reactive statements; days wasn't updated before save read it)
- also patch collection.itinerary order values so reactive rebuild uses new order
- replace single google_maps <option> with 8 AI LLM provider options
  (anthropic, openai, gemini, ollama, groq, mistral, github_models, openrouter)
- add getApiKeyProviderLabel() helper for saved key display with google_maps fallback
- add i18n keys for all new provider labels in en.json and de.json

frontend/src/locales/de.json

@@ -748,6 +748,14 @@
 		"no_api_keys_saved": "Noch keine API-Schlüssel gespeichert.",
 		"add_api_key": "API-Schlüssel hinzufügen",
 		"provider": "Anbieter",
+		"api_key_provider_anthropic": "Anthropic",
+		"api_key_provider_openai": "OpenAI",
+		"api_key_provider_gemini": "Google Gemini",
+		"api_key_provider_ollama": "Ollama (Lokal)",
+		"api_key_provider_groq": "Groq",
+		"api_key_provider_mistral": "Mistral AI",
+		"api_key_provider_github_models": "GitHub Models",
+		"api_key_provider_openrouter": "OpenRouter",
 		"api_key_provider_google_places": "Google Places API",
 		"api_key_value": "API-Schlüssel",
 		"api_key_value_placeholder": "Geben Sie Ihren API-Schlüssel ein",